Heart is the main organ in the circulatory system that circulates blood to all the parts of the body. It has four chambers that are surrounded by three thick muscular walls, i.e., epicardium, myocardium and endocardium.
The heart in humans is four-chambered. The two upper chambers of the heart are called the right and left atria while the two lower chambers are called right and left ventricles. The right and the left atria are separated by the inter-atrial septum and the two ventricles are separated by a wall called the inter-ventricular septum. The two atria receive blood from the body, while the ventricles pump blood to the body. The openings between the upper and lower chambers are controlled by valves that help the blood to flow in only one direction. The vessels that carry the pumped blood away from the heart are aorta and pulmonary artery while the pulmonary vein and the vena cava bring blood from the body into the heart.